What propeller chord is?

A propeller chord is a line defined on a single blade section. A propeller blade is a wing in rotation, so each section carries an aerofoil shape. Take a thin slice through the blade at any radius, and that aerofoil shape appears.

The chord line runs from that section's leading edge to its trailing edge. The chord length is the length of that line. Both terms appear in propeller datasheets, and builders use both.

A datasheet that lists "c at 0.7R" gives the chord length at the 70 percent radius station. That single number does not describe the whole blade. Each radial station has its own chord line and its own chord length.

The chord anchors every other geometric value on the blade. Pitch angle is measured from the propeller chord line. Angle of attack is the chord's tilt against the oncoming air, and camber is the curve above it.

Where one number must stand in for the whole blade, designers use the mean aerodynamic chord. The MAC is a length-weighted average across all blade stations. It surfaces in performance calculations, in propeller test reports, and in aerodynamic models that treat the rotor as a single equivalent aerofoil.

How propeller chord is measured

Propeller chord length is measured along the chord line at a station. The station is named by its radius from the hub, as a fraction of the tip radius. The 70 percent station, written 0.7R, is the convention for a single representative value.

Designers pick 0.7R because that band of the blade carries the highest aerodynamic load at typical RPM. The aerofoil there spins fast enough to generate strong lift without the compressibility losses that hurt the tip. A single chord length at 0.7R captures the working part of the blade.

Units follow the size convention the propeller is sold under. A 5 inch FPV propeller carries chord values in inches, while a 22 inch agri propeller reads in millimetres on Indian datasheets. The 4-digit code propellers (5045, 6030) blend the two, with chord values reported in mm by Asian manufacturers.

For a quick hands-on check, lay a propeller flat and measure across its widest point with a vernier or steel rule. That value is the maximum chord, somewhere between the mid-span and the 70 percent station. It is not the chord at the root, the tip, or the mean.

Full design or test work needs more than one number. Engineers report a chord distribution: a table or curve of chord length at every station from root to tip. NASA's OpenVSP defines blade geometry through a chord curve at each station.

How propeller chord varies along the blade

Propeller chord changes along the blade, almost never sitting at one constant value. On a drone propeller, chord typically rises from the root, peaks somewhere in the outer mid-span, and tapers towards the tip. The shape of that curve is called the propeller chord distribution.

Four drivers shape the chord curve along a drone blade. Outer stations spin faster and generate more lift per unit chord, so a smaller chord delivers the needed force there. The blade twists from root to tip, which lets the outer aerofoil run at a similar angle of attack with less chord.

The third driver is the Reynolds number along the blade. At the tip the local Reynolds number is high, so a slimmer chord is enough. Near the root the section sees slower air and needs a wider chord for useful lift.

Structural strength is the fourth driver of chord shape. The root has to carry the bending and centrifugal load of the outboard blade, so it needs a wider, thicker section. The tip carries little load below it, so it tapers to a slim aerofoil that reduces drag.

The result is a chord curve that builders see plotted on engineering datasheets and in tools like NASA OpenVSP. The maximum chord typically lands in the outer mid-span on common drone propellers. FPV race propellers skew slimmer across the whole span, while heavy-lift and agri propellers carry wider chord across the outer blade.

How propeller chord affects thrust and drag

Chord controls how much lifting area sits at each station of the blade. At a given RPM, doubling the chord roughly doubles the lift produced at that section. So a wider chord generates more thrust per blade.

Two costs accompany the wider chord on a blade. The first is drag: extra aerofoil area in the airflow brings higher profile drag, higher induced drag, and heavier current draw. The second is mass: a wider blade carries extra material, which raises rotational inertia and slows throttle response.

A narrower chord gives the opposite trade across the blade. Lift per blade drops, drag drops with it, and the motor stays cooler at the same RPM. To make up the lost thrust, the designer raises RPM, adds extra blades to the propeller, or accepts a smaller payload.

The split between these two strategies shows up across drone classes. FPV racing props run slim chord across the whole span and spin past 25,000 RPM, betting on raw RPM to deliver thrust. Agri and heavy-lift props carry wider chord across the outer blade and spin slower, betting on chord area to deliver thrust.

Chord also drives a blade's stall behaviour at high load. A wider chord at a given angle of attack flows attached air over more of the section before the boundary layer separates. A narrower chord stalls sooner, which limits how hard the propeller can be loaded before thrust collapses.

Across all blades on the propeller, the chord aggregates into the solidity ratio. The solidity ratio is the total blade area divided by the disk area, and it sets the propeller's whole-disk lift and noise signature.

From here, chord meets blade twist along the span and solidity ratio across all blades, three values that decide how a propeller flies.